Post A Phone: the no-frills, mailable landline

Sure, we're well aware that essentially any landline phone is mailable per se, but Priestman Goode's Post A Phone barely requires more than a single first-class stamp. Granted, it's still a concept for now, but this creation would be created from recyclable cardboard or plastic, have the ability to "pop out of a A5 envelope" and measure in at just four-millimeters in depth. Designed to act as a "fail safe and simple guarantee of communication," this disposable landline device should be just the thing for traveling spies and undercover two-timers.